The present invention relates to a new and improved thermal monitoring apparatus for an electrical component, such as a transformer winding or choke coil.
In its more particular aspects the present invention relates specifically to a new and improved thermal monitoring apparatus for a transformer or choke coil provided with forced circulation cooling, especially forced circulation oil cooling and a forced flow of the cooling medium through the transformer winding or the choke coil. The thermal monitoring apparatus comprises a container or vat filled by the cooling medium and a thermosensor including a thermosensor winding surrounding the thermosensor. The thermosensor winding is supplied via a current transformer with a current which is essentially proportional to the current flowing through the transformer winding or choke coil.
Transformers provided with forced oil cooling have known advantages, but also have the disadvantage that in the case of failure of the oil pumps and thus of the forced oil cooling of the windings of the electrical component to be monitored, the cooling of the windings is strongly impaired. In conformity with emergency procedures or operational regulations the measures to be undertaken in the event of such a defect or malfunction entail a reduction or cut-off in the power supply so as to prevent either the thermal destruction or the electrical destruction of the transformer, the latter being a direct consequence of gas development. When the transformer station constitutes an unmanned transformer station such objectives must be carried out by appropriate protective devices.
Presently, the so-called "thermal replica or copy" is still preferably employed for monitoring the thermal state of large-size transformers. A temperature sensor or feeler is placed in the oil which has the highest temperature and superposed thereon by means of a heater winding is the calculated hot spot temperature difference or surge between winding and oil. Depending upon the kind of apparatus, the temperature sensor may form a resistance wire for remote indication or a liquid-filled chamber for direct indication including control contacts. The heater winding is supplied with a current which is proportional to the current flowing in the transformer or choke coil by means of a "thermal replica transformer" and is intended to be matched to the thermal behavior of the transformer or choke coil which is to be monitored. In most cases the latter requirement is only approximately fulfilled since it is impossible for practical reasons to take account of the multitude of parameters when using a single heater winding. Finally, the aforementioned hot spot temperature difference or surge is adjusted with regard to a calculated or measured operative state by means of a variable current flowing through the heater winding by means of a parallel connected resistor. This kind of adjustment does not change the thermal resistance of the heater winding. Consequently, it will be recognized that the thermal replica or copy is only able to correctly indicate the final value of the temperature difference or surge without specific adaptation to the thermal resistance of the transformer winding or choke coil to be monitored, however, is unable to indicate the actual temperature pattern or variation as a function of load variation within the range of 2 to 3 time-constants of the winding.
As mentioned hereinbefore the thermal replica is well suited to indicate the thermal state during standard operation of transformers which are cooled in such a manner. However, in the case of failure of all of the oil circulation pumps the thermal parameters of the windings are drastically changed. On the one hand, the heat transmission resistance increases to a multiple of the value which can be attained using forced oil flow. On the other hand, there develops a large temperature spread or differential along the winding to be cooled in order to just maintain only a small natural oil flow through the relatively high hydraulic resistances formed by the cooler and the winding. This ultimately pronounced increase in the hot spot temperature is not indicated by the normal thermal replica since the same is adjusted to the thermal resistance of the winding existing during standard or normal operation.